One way to achieve near-100 percent network uptime is to use Hot Standby Routing Protocol (HSRP), a proprietary protocol from Cisco that is described in IETF (Internet Engineering Task Force) document RFC (Request for Comments) 2281, dated March 1998. The term Backup Routing Protocol is used to refer to a class of backup routing protocols including HSRP.
Cisco's Hot Standby Routing Protocol (HSRP) is a protocol that is layered on top of the UDP (User Datagram Protocol) layer in the network stack. HSRP is implemented in Cisco routers or route switch modules (RSMs) to provide failover and/or load balancing for Layer 3 routing. In particular, HSRP can provide automatic router backup when it is configured on Cisco routers that run the Internet Protocol (IP) over Ethernet, Fiber Distributed Data Interface (FDDI), and Token Ring local-area networks (LANs). For IP, HSRP allows one router to automatically assume the function of a second router if the second router fails. HSRP is useful for example when the users on one subnet require continuous access to resources in the network.
Cisco's HSRP can provide a way to keep communicating when a router becomes unavailable. HSRP allows two or more HSRP-configured routers to use the MAC (Medium Access Control) address and IP network address of a virtual router. The virtual router does not physically exist; instead, it represents the common target for routers that are configured to provide backup to each other. When HSRP is configured on a router, the router automatically selects one of the virtual MAC addresses from a range of addresses in the Cisco IOS software that is within the range of Cisco's MAC address block. Ethernet and FDDI LANs use one of the preassigned MAC addresses as a virtual MAC address. Token Ring LANs use a functional address as a virtual MAC address.
HSRP also works when the hosts are configured for proxy ARP (Address Resolution Protocol). In the event an active HSRP router receives an ARP request for a host that is not on the local LAN, the router replies with the MAC address of the virtual router. If the active router becomes unavailable or its connection to the remote LAN goes down, the router that becomes the active router receives packets addressed to the virtual router and transfers them accordingly.
Multigroup HSRP (MHSRP) is an extension of HSRP that allows a single router interface to belong to more than one Hot Standby group. MHSRP uses Cisco IOS Software Release 10.3 or later and is supported on routers that have special hardware that allows them to associate an Ethernet interface with multiple unicast Media Access Control (MAC) addresses. These routers are the AGS and AGS+ routers and any router in the Cisco 7000 series. The special hardware allows a user to configure a single interface in an AGS, AGS+, or Cisco 7000 series router so that the router is the backup router for more than one Hot Standby group.
In both HSRP and MHSRP, a tracking feature can be used to adjust the Hot Standby priority of a router based on whether certain of the router's interfaces are available. A “tracked interface” is a monitored interface between a back end of a group and some port of a network, e.g. it is an interface that is not internal to the group. When a tracked interface becomes unavailable, the HSRP priority of the router is decreased, for example because unavailability of the interface makes the router less useful. Tracking can be used to automatically reduce the likelihood that a router that already has an unavailable key interface will become the active router. To configure tracking, the “standby track” interface configuration command can be used.
HSRP or MHSRP can be used when configuring load sharing. For example where Routers A, B connect to a Local Area Network (LAN) 1.0.0.0 via Ethernet interfaces 0 respectively having addresses 1.0.0.1 and 1.0.0.2, and each of the Routers A, B also connects via a different interface (for example, a serial interface) to an IP network or internetwork. In this example, the Router A is configured as an Active router for a group 1 and as a Standby router for group 2, and the Router B is configured as a Standby router for group 1 and as an Active router for group 2. Half of the workstations on the LAN are configured for Router A, and half of the workstations are configured for Router B. Together, the configuration files for Routers A and B establish two Hot Standby groups. For group 1, Router A is the default active router, and Router B is the standby router. For group 2, Router B is the default active router, and Router A is the standby router. During normal operation, the two routers share the IP traffic load. When either router becomes unavailable, the other router becomes active and assumes the packet-transfer functions of the router that is unavailable. Interface configuration commands are used so that if a router goes down and then comes back up, preemption occurs and restores load sharing.
HSRP can be used with Routed Protocols such as AppleTalk, Banyan VINES, Novell IPX, DECnet and XNS. For example, HSRP can be configured in networks that, in addition to IP, run AppleTalk, Banyan VINES, and Novell IPX. AppleTalk and Novell IPX continue to function when the standby router becomes the active router, but they take time to adapt to topology changes.
In summary, HSRP and MHSRP use fault-tolerant routing of IP packets for networks in an effort to provide nonstop access by hosts on all segments to resources on all segments. To provide fault tolerance, HSRP and MHSRP use a routing protocol that converges rapidly, such as Enhanced Interior Gateway Routing Protocol (Enhanced IGRP).